First, break down the flashlight and remove the battery pack. You won't need it.

Remove the circuit board, silver spacer, and LED board from the front section. You'll have to unscrew the metal washer-type piece inside to remove everything.

Remove the rear button completely. Before installing your new switch, you'll want to cut down one of the two prongs on the back so that it will not make a connection with the battery. Only one of the prongs needs to touch the battery. The long one will be your positive, and the short one will be your negative. Then, take your replacement momentary switch and feed the button through the existing hole. Once it's flat with the rear of the case, put the lock washer and nut on the back side of the switch so it's held firmly in place. Solder a thin wire to the side of the shortened prong. This wire will go through the notch in the pvc, and, when the flashlight is assembled, it will rest between the pvc and the flashlight casing, creating the negative connection.

With a soldering gun, break the solder on the circuit board spring, as well as all the led connections. Make sure that you're being careful to not push the solder around on the circuit board. You want to make sure the solder points on the outer ring and the solder points inner ring of the board don't connect. This will ground the circuit out.

Once you've done that, you're able to solder the spring back in place on the circuit board. Use the small gauge solder. You don't want to make a mess.

Cut a piece of PVC so that it will extend from the back of the rear section (where the momentary switch is) to the metal spacer that will be above the circuit board. You can always adjust the length as you go. This will be the battery holder. It works great and holds them in place well. You might need to sand down the end of the PVC that will go inside the section with the switch. It's very tight. You'll also need to cut a notch on it that end so you can lead a wire from the connection you cut short on the momentary switch. This will be your negative connection.

Take your printed circuit board (or whatever you'll be mounting your connection to) and trace out the circle size you need. The best way to do that is to use the plastic lens that you removed to start. That'll provide the best fit. Place your 901 battery connector near the middle of the circuit board you just cut, and trace around it. Find a drill bit that is a shade smaller than that to do your drilling. This will provide that you get the snuggest fit. Once you're able to push the connector into the material, use JB Weld to secure it in place. Make sure you're not getting the JB Weld or liquid cement in or around the edges of the connection. These need to be clean so that you can later solder your connecting wires.
Take your telephone wires, feed one through the outer band of the circuit board, and solder it into place. Then, take your 2nd wire, and feed it through one of the inner band holes, and solder it in, as well. The wire in the inner band is your positive, and the outer band is your negative.

Cut those wires to an appropriate length, feed them through the holes in the silver spacer that came in the flashlight. Once you've done that, you will solder your negative wire to the outer ring of your battery connector, and the positive wire to the inner ring that surrounds the vent hole. You can see this process in madog's video very clearly.

Place the printed circuit board with your connector, the plastic spacer, and the circuit board into the flashlight. Place additional solder on the four outer ground spots on the circuit board, so that the outer ring of the board will be connecting to the flashlight casing. This completes the negative end of the circuit.

Once you've done this, you're read for the full assembly.

It's very hard to describe this entire process without pictures or video, but I hope I did a decent enough job. If you get stuck, or need any help, feel free to ask, and I'll do what I can to help.

More experienced builders: You're more than welcome to chime in with advice and guidance. I certainly don't know everything, and you might have much better advice than I do. It is most welcome!